What Is Stellite? Hardfacing Alloy
Stellite is a family of cobalt-chromium-tungsten hardfacing alloys used for valve seats, gates, plugs, and other wear-resistant surfaces in oil and gas piping. Stellite 6 (Co-28Cr-4.5W-1.2C) is the most common grade, applied as a weld overlay on valve sealing surfaces to resist galling, erosion, and corrosion at temperatures up to 800°C. Stellite is not a structural material; it is a surface treatment applied to valve trim and other components subject to metal-to-metal contact.
Common Stellite Grades
| Property | Stellite 6 | Stellite 12 | Stellite 21 |
|---|---|---|---|
| AWS classification | ERCoCr-A | ERCoCr-B | ERCoCr-E |
| Cobalt | Balance (~58%) | Balance (~54%) | Balance (~59%) |
| Chromium | 27.0-31.0% | 28.0-32.0% | 26.0-29.0% |
| Tungsten | 4.0-6.0% | 7.5-9.5% | - |
| Molybdenum | 1.5% max | 1.5% max | 4.5-6.5% |
| Carbon | 0.9-1.4% | 1.1-1.7% | 0.20-0.35% |
| Nickel | 3.0% max | 3.0% max | 2.75% max |
| Iron | 3.0% max | 3.0% max | 3.0% max |
| Hardness (as-deposited) | 38-44 HRC | 42-50 HRC | 28-34 HRC |
| Max service temp | 800°C | 800°C | 800°C |
Why Stellite for Valve Trim?
Valve sealing surfaces (seats, gates, discs, plugs, and balls) undergo repeated metal-to-metal contact during opening and closing cycles. Without hardfacing, stainless steel and carbon steel seats gall (seize) and wear, leading to leakage. Stellite provides:
- Galling resistance: the cobalt-chromium matrix resists adhesive wear and seizure during metal-to-metal contact
- Erosion resistance: hard carbide particles in the cobalt matrix resist particle erosion from process fluids
- Hot hardness: Stellite retains hardness above 30 HRC at 600°C, unlike most tool steels that soften above 500°C
- Corrosion resistance: high chromium content provides oxidation resistance comparable to stainless steel
Grade Selection
Stellite 6 is the default for most valve applications: gate valve seats and wedges, globe valve discs and seats, check valve flappers and seats, and ball valve seats. It balances wear resistance with ductility and is weldable onto most valve body materials including carbon steel, alloy steel, and stainless steel.
Stellite 12 is specified for higher-wear applications: throttling service globe valves, control valves with frequent cycling, and severe erosion conditions. The higher carbon and tungsten content produces more carbides and higher hardness.
Stellite 21 is the choice for high-temperature nuclear and high-impact applications. Lower carbon content provides better ductility and thermal shock resistance. The molybdenum (instead of tungsten) reduces susceptibility to intermetallic precipitation at elevated temperatures.
Application Methods
| Method | Typical Thickness | Advantages |
|---|---|---|
| PTA (Plasma Transferred Arc) | 1.5-3.0 mm | Precise, low dilution, consistent quality |
| GTAW (TIG) | 1.5-3.0 mm | Versatile, field-repairable |
| Laser cladding | 0.5-2.0 mm | Very low dilution, minimal HAZ |
| Oxy-fuel (powder) | 1.5-3.0 mm | Simple, suitable for large surfaces |
Minimum two passes are typical; the first pass has high dilution from the base metal, while the second achieves the specified Stellite chemistry and hardness.
Specifications and Standards
Stellite hardfacing for valve trim is specified in API 600 (gate valves), API 602 (small-bore valves), API 623 (globe valves), and ASME B16.34. The hardfacing material, thickness, and hardness must be documented on mill test certificates. NDT includes liquid penetrant examination of the overlay surface to detect cracks and porosity. Hardness testing verifies the deposit meets the specified HRC range.
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